The present invention relates to a spin-orbitronics device, and more particularly to a device whose operation is effectuated by spin transfer torque (STT) and spin-orbit interaction.
A magnetic tunnel junction (MTJ), which normally includes a magnetic free layer and a magnetic reference layer with an insulating tunnel junction layer interposed therebetween, serves as the memory element for a new class of non-volatile memory known as magnetic random access memory (MRAM). The magnetic reference layer has an invariable or fixed magnetization direction, while the magnetic free layer has a variable or switchable magnetization direction that is parallel or anti-parallel to that of the magnetic reference layer. When the magnetization directions of the magnetic free and reference layers are substantially parallel to each other, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel or opposite to each other. Accordingly, an MTJ has two stable resistance states that may be used as a memory element to store one bit of logical data.
Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, whose magnetization directions lie within planes substantially parallel to the same layers, or perpendicular MTJ, whose magnetization directions are substantially perpendicular to the layer planes.
The magnetization direction of the magnetic free layer and thus the resistance state of the MTJ may be switched by a spin-polarized electron flow that exerts a spin transfer torque on the magnetic free layer. FIGS. 1A and 1B illustrate switching of the magnetization direction of the magnetic free layer of a perpendicular MTJ 50. The perpendicular MTJ 50 includes a magnetic reference layer 52 having an invariable perpendicular magnetization direction 54 and a magnetic free layer 56 having a variable perpendicular magnetization direction 58 or 58′ with an insulating tunnel junction layer 60 interposed therebetween. FIG. 1A illustrates switching of the magnetization direction 58 of the magnetic free layer 56 from anti-parallel to parallel configuration as a parallelizing electron flow 62 passes from the magnetic reference layer 52 to the magnetic free layer 56. As electrons pass through the magnetic reference layer 52 and are being spin-polarized, the spin-polarized electrons exert a spin transfer torque (STT) on the magnetic free layer 56, causing the magnetization direction 58 of the magnetic free layer 56 to switch from the anti-parallel to parallel configuration when the spin-polarized electron flow exceeds a threshold.
Conversely, FIG. 2B illustrates switching of the magnetization direction 58′ of the magnetic free layer 56 from parallel to anti-parallel configuration as an anti-parallelizing electron flow 64 passes from the magnetic free layer 56 to the magnetic reference layer 52. As electrons pass through the magnetic free layer 56, the electrons with the same spin orientation as that of the magnetization in the magnetic reference layer 52 pass into the magnetic reference layer 52 unimpeded. However, the electrons with the opposite spin orientation are reflected back to the magnetic free layer 56 at the boundary between the tunnel junction layer 60 and the magnetic reference layer 52, causing the magnetization direction 58′ of the magnetic free layer 56 to switch from parallel to anti-parallel configuration when the anti-parallelizing electron flow 64 exceeds a threshold level.
Similarly, FIGS. 2A and 2B illustrate switching of the magnetization direction of the magnetic free layer of an in-plane MTJ 66. The in-plane MTJ 66 includes a magnetic reference layer 68 having an invariable in-plane magnetization direction 70 and a magnetic free layer 72 having a variable in-plane magnetization direction 74 or 74′ with an insulating tunnel junction layer 76 interposed therebetween. Like the perpendicular MTJ 50, the magnetization direction 74 of the magnetic free layer 72 may be switched to the parallel configuration by passing a parallelizing electron flow 78 from the magnetic reference layer 68 to the magnetic free layer 72 as shown in FIG. 2A. The magnetization direction 74′ of the magnetic free layer 72 may be switched to the anti-parallel configuration by passing an anti-parallelizing electron flow 80 from the magnetic free layer 72 to the magnetic reference layer 68 as illustrated in FIG. 2B. The application of conventional MTJs, such as those shown in FIGS. 1A, 1B, 2A, and 2B, is mostly limited to non-volatile memory because of their limited capabilities.
Recently, several studies (see for example, Mellnik et al., Nature, 511, 449-451 (2014); Cubukcu et al., Applied Physics Letters, 104, 042406 (2014); Miron et al., Nature, 476, 189 (2011); Avci et al., Applied Physics Letters, 100, 212404 (2012); Liu et al., Science, 336, 555 (2012)) have demonstrated the switching of a magnetic free layer in contact with a non-magnetic heavy metal layer or a topological insulator layer by a spin-orbit torque generated when electrons are injected into the non-magnetic metal layer. Experimental evidence suggests that the injection of electrons generates a perpendicular spin current via spin-orbit interaction that transfers to the magnetization of the magnetic free layer to generate a spin torque and reverse the magnetization thereof. It is believed that the spin torque may be generated by spin Hall effect or Rashba-like interface effects or both. The devices proposed in the above-mentioned studies, however, can only switch the magnetic free layer. Therefore, like the conventional STT-MTJ described above, the applications of the proposed devices are limited to conventional memory devices with simple read and write functions.
For the foregoing reasons, there is a need for a spin-orbitronics device that can have additional functions and that can be inexpensively manufactured.